IR Spectroscopy

Infrared light goes from 400 to 4000 $\mathrm{c}{\mathrm{m}}^{-1}$, or

Zones of IR Spectroscopy

1. The Polar $\mathrm{H}$‘s
2. The $\mathrm{C}-\mathrm{H}$‘s
3. The Triple Bonds
4. The Carbonyls
5. The Double Bonds

Bond	Stretching Frequency ($\mathrm{c}{\mathrm{m}}^{-1}$)	Intensity and Shape	Zone
Alcohol $\mathrm{O}-\mathrm{H}$	3650-3200	Usually strong and broad	1
Alkyne $\mathrm{C}\equiv \mathrm{H}$	3340-3250	Usually strong and sharp	1
Amine or amide $\mathrm{N}-\mathrm{H}$	3500-3200	Medium; often broad	1
Aryl or vinyl $s{p}^2\mathrm{C}-\mathrm{H}$	3100-3000	Variable	2
Alkyl $s{p}^3\mathrm{C}-\mathrm{H}$	2960-2850	Variable	2
Aldehyde $\mathrm{C}-\mathrm{H}$	~2900, ~2700	Medium; two peaks	2
Carboxylic acid $\mathrm{O}-\mathrm{H}$	3000-2500	Usually strong; very broad	2
Alkyne $\mathrm{C}\equiv \mathrm{C}$	2260-2000	Variable and sharp	3
Nitrile $\mathrm{C}\equiv \mathrm{N}$	2260-2220	Variable and sharp	3
Ketone $\mathrm{C}=\mathrm{O}$	1750-1705	Strong	4
Ester $\mathrm{C}=\mathrm{O}$	1750-1735	Strong	4
Aldehyde $\mathrm{C}=\mathrm{O}$	1740-1720	Strong	4
Carboxylic acid $\mathrm{C}=\mathrm{O}$	1725-1700	Strong	4
Amide $\mathrm{C}=\mathrm{O}$	1690-1650	Strong	4
Alkene $\mathrm{C}=\mathrm{C}$	1680-1620	Variable	5
Benzene $\mathrm{C}=\mathrm{C}$	~1600; ~1500-1450	Variable; 1600 often two peaks	5
Below 1400 $\mathrm{c}{\mathrm{m}}^{-1}$ is the fingerprint region, which mostly contains bending vibrations and unfortunately a lot of noise

IR Spectroscopy Chart

$\bar{\omega }=\frac{1}{2\pi c}\sqrt{\frac{k}{\mu }}$

• The stretching vibrational frequency of light
• $c$ is the speed of light
• $k$ is the spring constant
• $\mu$ is the reduced mass